1. Field of the Invention
This invention relates to a developing apparatus for developing electrostatic images formed on an image carrier using, e.g., an electrophotographic method, an electrostatic recording method and, more particularly, to an apparatus used for photocopiers, printers, and facsimile machines.
2. Description of Related Art
Conventionally, color image forming apparatuses have been well known in which having plural developing apparatuses containing respectively developers of different colors and obtaining multicolor images using electrophotographic methods.
Referring to FIG. 3, described is a main cross-sectional structure of a conventional multicolor image forming apparatus. Exemplified herein as a conventional multicolor image forming apparatus is that four-color developing apparatuses are contained in a rotary body, which rotates by a prescribed angle at developing operation of respective colors to develop properly the images of respective colors.
In this prior art, the developing apparatus has a feature equipped with two developing rollers for respective colors, and the developer contains a two-component made of a so-called carrier serving as a magnetic agent and a toner for actually developing the latent image.
In this color image forming apparatus, an image processing means is constituted of a photosensitive drum 1 serving as an image carrier, a primary charger 2 as a charging means, a laser scanner 3 providing image information, a rotary type developing apparatus 5 supporting developing devices 4a, 4b, 4c, 4d as four developing means, an intermediate transfer body 6, and a cleaning apparatus 7 collecting remaining transfer toner.
The image processing means ensures a size in the longitudinal direction corresponding to a prescribed length in the longitudinal direction of the photosensitive drum 1 required for image formation of a prescribed width.
The rotary type developing apparatus 5 is of a type settling one of four developing devices 4a to 4d to a developing position according to the rotation of a supporting body 9. The rotary type developing apparatus 5 has development opening surfaces for the respective developing devices 4a to 4d on a circumference of a circle whose center is at a support body rotary shaft 8.
The developing devices 4a to 4d have first developing rollers 10a, 10b, 10c, 10d and second developing rollers 11a, 11b, 11c, 11d serving as rotary developer carriers for conveying the toner to a portion contacting to the photosensitive drum 1.
The rotary type developing apparatus 5 has a structure that only during the developing operation the first developing rollers 10a, 10b, 10c, 10d and the second developing rollers 11a, 11b, 11c, 11d in the developing devices 4a to 4d are made closer to the photosensitive drum 1 within a prescribed amount by moving the developing devices 4a to 4d with positioning apparatuses 12a, 12b, 12c, 12d and that during the rotation of the rotary type developing apparatus 5 as well as during non-development operation the first developing rollers 10a, 10b, 10c, 10d and the second developing rollers 11a, 11b, 11c, 11d in the developing devices 4a to 4d are isolated from the photosensitive drum 1 by releasing the contacting operation of the positioning apparatuses 12a, 12b, 12c, 12d. 
Referring to FIG. 4 and FIG. 5, the developing device positioning apparatus for the rotary type developing apparatus in FIG. 3 is described. Numeral 20 denotes a photosensitive drum; numeral 21 denotes a supporting body of a rotary type developing apparatus 22; numeral 23 denotes a developing device. The developing device 23 has mounted therein a first developing roller 24 and a second developing roller 25. The developing device 23 is installed movably on a developing guide 26. The developing guide 26 is set so that the first developing roller 24 and the second developing roller 25 are movable parallel with the surface of the photosensitive drum 20 up to coming closer in a prescribed amount. Numeral 27 is a positioning cam and moves pivotally around a pivotal shaft 28. A roller 29 is disposed rotatably at a tip thereof.
The rotary type developing apparatus 22 rotates the developing device 23 by a rotary driving means, not shown, to a prescribed position on the photosensitive drum 20 at which the electrostatic latent image can be developed. The positioning cam is rotated in arrow B direction in the drawing with a drive apparatus, not shown, around the pivotal shaft 28. According to this rotation, the developing device 23 moves in arrow C direction in the drawing and reaches the state shown in FIG. 5.
In FIG. 5, a part of a container 30 hits a hitting member, not shown, of the supporting body 21 to cease the developing device 23, so that the developing device 23 is held as it is according to a holding force of the positioning cam 27. In this state, the first developing roller 24 and the second developing roller 25 of the developing device 23 can come closer to the prescribed amount such that the electrostatic latent images on the photosensitive drum 20 can be properly developed. The prior art has a structure that this prescribed amount is in a range from 200 microns to 400 microns.
Next, a process for forming color images on a transfer material using this color image forming apparatus is described.
The photosensitive drum 1 is rotated in arrow A direction, and the primary charger 2 charges the photosensitive drum 1 uniformly. Exposure 13 by the laser scanner 3 then forms electrostatic latent images on the photosensitive drum 1. These latent images are corresponding to color developing agent developers in the developing devices 4a to 4d, or namely respective colors such as yellow (Y), magenta (M), cyan (Y), and black (Bk).
Regarding to the first color, the electrostatic latent image corresponding to, e.g., the yellow developer is formed on the photosensitive drum 1, and the image, after being visualized with the known process among the developer rollers 10a, 11a of the developing device 4a in which the yellow developer is contained, and the photosensitive drum 1, is transferred with a first transfer roller 14 to the intermediate transfer body 6.
Subsequently, after the remaining toner on the photosensitive drum 1 is cleaned up with the cleaning apparatus 7 such as a blade or the like, the electrostatic latent image corresponding to, e.g., the developer, as the second color, is formed on the photosensitive drum 1, and the image, after being visualized with the known process among the developer rollers 10b, 11b of the developing device 4b in which the magenta developer is contained, and the photosensitive drum 1, is transferred in an overlapping manner with the first transfer roller 14 to the intermediate transfer body 6 to which the yellow visualized image of the first color is transferred.
After the above operations are done in a plural number, the toner overlapped as plural layers on the intermediate transfer body 6 is transferred onto a transfer material 102 as described below.
Hereinafter, the step that the toner of the plural layers on the intermediate transfer body 6 is transferred onto the transfer material 102 is described.
A transfer material 102 stacked on a cassette 101 in FIG. 3 is fed out with a pickup roller 103, and the topmost transfer material P2 is solely transferred on a downstream side by a feeding roller 104 and a reverse separation roller 105 rotating in the reverse direction to the feeding direction. The transfer material P2 is then conveyed along a conveyance path by plural conveyance roller pairs 106, and is bent upon the front end's engagement with a nipping portion of a stopped registration roller pair 107, thereby being corrected from obliquely feeding.
The registration roller pair 107 begins rotating at a timing synchronized with a toner image formed on the intermediate transfer body 6, thereby feeding out the transfer material 102. The fed transfer material 102 enters in the nipping portion formed with the intermediate transfer body 6 and a second transfer roller 108, and the toner image on the intermediate transfer body 6 is transferred to the transfer material (P1 in FIG. 3).
The transfer material P1 is then conveyed with a conveyance belt 109 to a fixing apparatus 110 made of two rollers, located on upper and lower sides. The toner image on the transfer material P1 at the fixing apparatus 110 is fixed to the transfer material P1, and is delivered to a delivery tray 112 via delivery rollers 111.
Such a color image forming apparatus has been proposed as an example of a color image forming apparatus particularly for higher image quality than the usual image quality. Such an apparatus has a feature that each of the four-color developing devices contained in the rotary body has two developing rollers. Described herein as an example of such an apparatus feature is that the four-color developing devices contained in the rotary body have the same shape and the same mass and are disposed equally with 90 degrees spaced with respect to a rotary center of the rotary body.
Next, the developing device in the rotary body is described.
FIG. 6 and FIG. 7 show two states taken by the developing device in a rotary body 40. In FIG. 7, numeral 41 shows a photosensitive drum, and FIG. 7 shows a situation that the electrostatic latent image on the photosensitive drum 41 is developed by an developer of a developing device 42. The rotary body 40 has a structure to rotate by each of 90 degrees in the counterclockwise direction (arrow D in the drawing). FIG. 6 shows the state that the rotary body 40 is located at a position of −90 degrees in the rotational direction of the rotary body 40 (rotational direction of the rotary body 40 is defined as “+”) with respect to the developing position shown in FIG. 7.
With reference to FIG. 6, an internal structure of the developing device 42 is described. The developing device 42 has a first developing roller 44 and a second developing roller 45, and each of the developing rollers 44, 45 has a first developing sleeve 46 and a second developing sleeve 47, which are in a rotatable state with a drive mechanism, not shown, and a first magnet roller 48 and a second magnet roller 49 stationally formed at respective centers of the developing rollers 44, 45.
As shown in FIG. 6, the magnetic polarity arrangement of the first magnet roller 48 and the second magnet roller 49 is such that the north polarity and the south polarity are set entirely reverse to each other.
In the developing device 42, a first screw 50 and a second screw 51 are disposed. A diaphragm 53 structured by a part of a container 52 is projected between the first screw 50 and the second screw 51, thereby forming a developing chamber 54 enclosed with the first developing roller 44 and a stirring chamber 55 enclosed with the second developing roller 45, respectively. The opposite ends in the longitudinal direction of the first screw 50 and the second screw 51 are supported with bearings, not shown.
For the purpose of the explanation, the space between the first developing roller 44 and the second developing roller 45 is defined as an SS space 56.
A situation shown in FIG. 6 is described. As described above, FIG. 6 shows the state of −90 degrees with respect to the developing position, and in this state, the first developing roller 44 and the second developing roller 45 are positioned on a substantially lower side, and therefore, a developer 57 existing in the developing device 42 is deposited as to overlap the first developing roller 44 and the second developing roller 45 by operation of the gravity force as shown with “circle patterns” in FIG. 6. From another viewpoint, the developing 57 developer is attracted by magnetic force of the first magnet roller 48 and the second magnet roller 49 in the first developing roller 44 and the second developing roller 45, and a space including the SS space 56, other than an empty space at an upper portion of the developing chamber 54 and the stirring chamber 55, is filled with the developer 57 by a synergy effect with the above gravity force.
Next, the state shown in FIG. 7 is described below. As described above, FIG. 7 shows a state for developing position, and in this state, the first developing roller 44 and the second developing roller 45 are in a drive state, while the first developing sleeve 46 and the second developing sleeve 47 rotate in directions of arrows E, F.
Regarding the first screw 50 and the second screw 51, as shown in the drawing in respect with the developer 57 in the developing device 42, the lead and rotational direction of the first screw 50 is set so that the developer 57 is conveyed in a direction toward the front side of the drawing, and similarly, the lead and rotational direction of the second screw 51 is set so that the developer 57 is conveyed in a direction toward the rear side of the drawing. The developing chamber 54 containing the first screw 50 and the stirring chamber 55 containing the second screw 51 are structured to communicate with each other at the opposite ends in the longitudinal direction.
Referring to FIG. 8, the flow of the developer 57 existing in the developing device 42 is described. FIG. 8 is a schematic view showing a cross section along an axial line extending in the longitudinal direction of a developing device 60. Numeral 61 denotes a first screw described in FIGS. 6, 7; numeral 62 denotes a second screw; numeral 63 denotes a developing chamber containing the first screw 61; numeral 64 denotes a stirring chamber containing the second screw 62. Numeral 65 denotes a toner hopper for supplying a toner 66 to the developing device 60.
One flow is for equalizing the toner density of the developer in the developing device 60. The toner 66 supplied from the toner hopper 65 is supplied to the developing device 60 from extension 641 of the stirring chamber 64 and conveyed in the stirring chamber 64 in arrow direction of arrows G, H as stirred with the carrier in the stirring chamber 64 and as maintaining the state of the developer. The toner 66 at that time receives further triboelectric charges from adequate stirring with the carrier.
The developer is overly filled at an end 642 of the stirring chamber 64 because the developer is sent to the second screw 62. The end 642 of the stirring chamber 64 is structured with a communication path 67 in communication with the end 632 of the developing chamber 63, so that overly filled developer enters toward a side of the end 632 of the developing chamber or first chamber 63.
The first screw 61 of the developing chamber 63 conveys the developer in the developing chamber or first chamber 63 in a direction of arrow I in the drawing. The developer conveyed in the developing chamber 63 reaches the other end 631 of the developing chamber 63. Another communication path 68 in communication with the stirring chamber or second chamber 64 is formed even at the other end 631 of the developing chamber 63, so that the developer is circulated into a merging portion 643 of the stirring chamber 64 again.
Other flows of the developer are used for a process developing the electrostatic latent image on the photosensitive drum 41 with the developer in the developing device 42. This flow is described with reference to FIG. 7.
As shown in FIG. 7, the developer 571 in the developing chamber 54 faces to the first developing roller 44, and a part of the developer 571 conveyed thereto is fed around the surface of the first developing sleeve 46 by magnetic attracting force of the first magnet roller 48 of the first developing sleeve 46, and is carried as a thin layer with a restricting blade 58. The developer 57 develops the electrostatic latent image on the surface of the photosensitive drum 41 according to the rotation of the first developing sleeve 46 and is further conveyed to the second developing roller 45.
As a feature of the magnetic polarity arrangement of the first magnet roller 48 and the second magnet roller 49, the magnetic flux density is made higher at a space between the N1 polarity 481 of the first magnet roller 48 and the S2 polarity 491 of the second magnet roller 49 because the N1 polarity 481 of the first magnet roller 48 and the S2 polarity 491 of the second magnet roller 49 are arranged as opposed to each other, so that transfer of the developer 571 can be done surely from the first developing sleeve 46 to the second developing sleeve 47, and so that the toner and the carrier can be prevented from scattered between the sleeves 46, 47.
The second developing roller 45 carries the developer 571 to the surface of the second developing sleeve 47 in substantially the same manner by the magnetic attracting force of the second magnet roller 49. The developer 571 on the surface of the second developing sleeve 47 performs the second development of the electrostatic latent image on the photosensitive drum 41. Accordingly the two developments with the first developing roller 44 and the second developing roller 45 render the developer 571 entering into the stirring chamber 55 in the developing device 42 again in a state that the toner density of the developer 571 is lowered. In the stirring chamber 55, the developer 572 is stirred and conveyed by the second screw 51, exists and merges with the developer 571.
In a meantime, as described above, almost all of the space including the SS space 56 is filled with the developer 57 by the synergy effect of the gravity force and the magnetic force of the two magnet rollers 48, 49.
If the rotary body 40 is rotated to a developing position shown in FIG. 7 from this state, the developer 57 in the developing device 42 is located in a range shown with “circle patterns” and “dot patterns” in the drawing.
Regarding the developer 571 of “circle patterns” in FIG. 7, as shown as the prior art, while the agent is circulated by the two developing screws 50, 51 in the developing device 42, a part of the agent is carried with two developing rollers 44, 45 to contribute the development of the electrostatic latent image on the photosensitive drum 41.
On the other hand, the portion shown with “dot patterns” in FIG. 7 is a portion defined as the SS space 56 in FIG. 6 and is a portion at which the developer 573 is carried by magnetic force of the two magnet rollers 48, 49. In this SS space 56, the S2 polarity 482 of the first magnet roller 48 and the N1 polarity 492 of the second magnet roller 49 are arranged as to be opposed to each other, so that the magnetic flux density is made higher between the polarities, and so that the SS space 56 is subject to higher constraint force against the developer 573 by the magnetic force.
The developer 573 is a agent that is circulated into the stirring chamber 55 after the developing operation of the previous time and that is contained in the stirring chamber 55. That is, the developer 573 is a developer with a reduced toner density, which is entirely different from the developer passing through the restricting blade 58 under the regular process. In the state shown in FIG. 6, the developer enters into the SS space 56.
The developer 573 carried in the SS space 56 is conveyed to the second developing sleeve 47 again as shown in arrow K in FIG. 7, and the coating amount of the developer of the second developing sleeve 47 greatly excesses the coating amount of the first developing sleeve 47 normalized with the restricting blade 58.
With such a state that the developer is mixed, not only defective image such as reduced density after duration may be formed, but also the coating amount of the second developing sleeve 46 itself becomes greatly larger than the proper amount, in the developing process of the electrostatic latent image on the photosensitive drum 41, thereby raising various problems such that blurs and overflows from the container 52 simply occur to scatter the toner in the device.
This invention is made in consideration of the above problems. It is an object to provide a developing apparatus avoiding a developer to be coated more than the necessary amount on a developing roller to prevent blurs and overflows out of a container form occurring and to obtain developed images with high quality.